1. Field of the Invention
The present invention relates to an image processing device, and image processing method, and a program, and more particularly relates to an image processing device, and image signal processing method, and a program, for performing signal processing as to output of an imaging device having an RGBW array.
2. Description of the Related Art
Imaging devices (image sensors) used for imaging apparatuses, for example, have a structure wherein a color filter, which transmits specific wavelength component light (R, G, B) in increments of pixels, is applied to the surface thereof. For example, a filter having an RGB array such as shown in FIG. 1A is used. With color image generating processing using output signals of an imaging device, processing is performed for reproducing color components to be used, in sets of multiple pixels. While there are various types of color arrays for color filters, the Bayer array shown in FIG. 1A, configured of three types of filters transmitting only the specific wavelengths of red (R), green (G), and blue (B), is widely used.
As of recent, imaging devices (image sensors) have come to have finer pixel configurations, and accordingly there has arisen the problem that the amount of incident light to each of the pixels decreases and the S/N ratio increases. In order to deal with this problem, there has been proposed an image sensor (imaging device) having, in addition to the filters which transmit only specific wavelength light of RGB or the like, white (W) which broadly transmits visible light, as shown in FIG. 1B. FIG. 1B illustrates an example of a filter having an RGBW array. The W pixels in the RGBW array shown in FIG. 1B are filters which broadly transmit visible light.
Such an imaging device having a color filter with white (W) pixels is described in, for example, US Patent Application Publication Nos. 2007/0024879 and 2007/0024934. Using an imaging device (image sensor) such as shown in FIG. 1B which has white (W) pixels raises the transmissivity of the filter, and higher sensitivity can be realized. However, RGBW type devices have the following problems.
Both the RGB array shown in FIG. 1A and the RGBW array shown in FIG. 1B are single-sensor image sensors where all or part of R, G, B, and W filters are arrayed in mosaic fashion on a single device. Accordingly, demosaicing processing, which is color coding in which RGB pixel values corresponding to the pixels has to be performed when generating a color image.
With the RGBW array shown in FIG. 1B, the sampling rate of the R, G, and B components deteriorates as compared to the RGB array shown in FIG. 1A. Consequently, in the event that data acquired with the RGBW array type device shown in FIG. 1B is used in color image generating processing, there is a problem that false color more readily occurs as compared with the RGB array shown in FIG. 1A. Also, the white (W) wavelength component includes all wavelength components of R, G, and B, so using an optical lens with great chromatic aberration can result in poorer light collection as compared with monochromatic arrangements, leading to deterioration in resolution. This problem becomes more marked as pixel configurations become finer.
As a technique to prevent deterioration in resolution due to chromatic aberration of the optical lens, combining lenses with different refractive indexes effectively prevents chromatic aberration from occurring, but this leads to a new problem of increased costs, since the number of lenses increases. This configuration also leads to a problem in that false color due to deterioration of the sampling rate of the RGB components described above is more marked.
Also, each pixel in a single-sensor image sensor only has monochromatic component information, so demosaic processing is performed in which RGB pixel values for all pixels are obtained to obtain a color image from the R, G, B, and W signals acquired discretely. At the time of demosaic processing, interpolation processing is performed which assumes that the color ratio will be generally constant in local regions and that there is a strong color correlation. Specifically, at the time of calculating pixel values for a particular color of a certain pixel, a method of performing interpolation using the pixel values of surrounding pixels is widely employed. An example of this technique is described in Japanese Unexamined Patent Application Publication No. 2009-17544. However, near the edges, the above assumption of the color ratio being generally constant in local regions and of strong color correlation does not hold. Accordingly, there is a problem that false color readily occurs near the edges.